1. Field of Invention
The invention relates, generally, to a vehicle independent suspension and, more particularly, to improvement of elasto-kinematic performance of the vehicle independent suspension architecture commonly known as “MacPherson architecture.”
2. Description of Related Art
The MacPherson suspension architecturt is fundamentally based on the idea of using the damper as an integrating part of the structure of the suspension. With referenceto FIGS. 1 and 2 of the attached drawings, which schematically show an example of vehicle front suspension accordnig to the MacPherson architecture, in perspective view and in front elevation, respectively, the suspension includes a damper 12 rigidly connected at its bottom end to a wheel-carryrng knuckle 22 (hereinafter simply referred to as “knuckle”), trianaular lower arm 14 having two transversely inner attachment points 16 and 18 for articulated connection to the vehicle body (indicated “B” in FIG. 2 ) and a transversely outer attachment point 20 for articulated connection to the knuckle 22, and a steering rod 24 having a transversely inner attachment point 26 for connection to the vehicle steering control mechanism and a transversely outer attachment point 28 for articulated connection to the knuckle 22. By virtue of the rigid connection between damper and knuckle 22 , the longitudinal and lateral forces acting on the wheel produce bending and shear stresses in the damper and are transmitted to the vehicle via the top mount of the damper.
The main advantages of the MacPherson architecture are the low cost and the small overall size. Since the functions of shock absorption, of provision of a mount for the spring and of reaction to the loads acting on the wheels are performed by the damper, it is in fact possible to obtain a considerable reduction in the number of components of the suspension, with resulting reduction in the cost thereof. Moreover, removing the upper lateral control arms leads to a reduction in the overall size of the suspension, which allows, in case of a front suspension, to obtain more space available for the engine, which is particularly advantageous in front-wheel drive motorcars with transversely-mounted engine and gearbox units.
The MacPherson architecture suffers however from the drawback of having low kinematic performances, with regard in particular to the camber change during the bump/rebound motion of the wheel. The performances of the MacPherson architecture in terms of load absorption are low, in particular with regard to the longitudinal forces acting on the wheel in case of braking or impact. More specifically, the longitudinal stiffness provided by the MacPherson architecture at the wheel center is much higher than that required to ensure good comfort performances. A further drawback of the MacPherson architecture is the reduced possibility to differentiate the toe change response of the suspension in traction and in braking Moreover, in the MacPherson architecture all the longitudinal compliance of the suspension is provided by the lower triangular arm, whose geometry is largely determined by the necessity to meet the requirement to combine high longitudinal compliance with high lateral stiffness, and therefore less freedom is left to the designer.
Additionally, according to the MacPherson architecture the point of maximum longitudinal stiffness of the suspension is located at the top mount of the damper, and accordingly the longitudinal stiffness of the suspension at the contact patch between the wheel and the road is much lower than that at the wheel center (in proportion to the vertical distances of these two locations from the top mount of the damper - point of maximum longitudinal stiffness), for example 50% to 75%. This has a detrimental effect on braking response. Since all the longitudinal compliance is provided by the lower arm, the ball joint between the lower arm and the knuckle undergoes significant longitudinal displacements, which results directly in castor loss in braking and hence in loss of stability of the vehicle. The practical result of this is that the designer is compelled to reduce the longitudinal compliance at the wheel center to avoid an excessive longitudinal compliance at the contact patch and hence excessive castor loss in braking.
It is therefore an object of the invention to provide a motorcar independent suspension which has better performances than the MacPherson architecture discussed above in terms of load absorption, particularly with regard to longitudinal forces, while retaining the advantages of this known architecture in terms of cost and overall size.